1. Field Of The Invention
The present invention is concerned with catalysts useful for the treatment of gases to reduce contaminants contained therein. More specifically, the present invention is concerned with improved catalysts which may function as catalysts of the type generally referred to as "three-way conversion" or "TWC" catalysts Whereas oxidation catalysts have the capability of catalyzing reactions such as the oxidation of hydrocarbons and carbon monoxide, TWC catalysts are polyfunctional in that they have the capability of substantially simultaneously catalyzing both oxidation and reduction reactions, such as the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides Such catalysts find utility primarily in the treatment of the exhaust gases from internal combustion engines, such as automobile and other gasoline-fueled engines.
2. Background and Related Art
In order to meet emissions standards for unburned hydrocarbons, carbon monoxide and nitrogen oxide contaminants in vehicle and other engine exhaust gases, catalytic converters containing oxidation catalysts, or oxidation and reduction catalysts, or a TWC catalyst, are emplaced in the exhaust gas line of internal combustion engines to catalytically promote the oxidation of unburned hydrocarbons ("HC") and carbon monoxide ("CO") and the reduction of nitrogen oxides ("NO.sub.x ") in the exhaust gas. TWC catalysts usually require that the ratio of air to fuel ("A/F ratio") introduced into the engine whose exhaust gas is being treated be at or within a narrow deviation from the stoichiometric A/F ratio in order to achieve good efficiencies of conversion of HC, CO and NOx pollutants to innocuous substances, that is, to carbon dioxide, water and nitrogen.
Known TWC catalysts which exhibit good activity and long life comprise one or more platinum group metals (e.g., platinum or palladium, preferably including one or more of rhodium, ruthenium and iridium, especially rhodium) distended upon a high surface area, refractory oxide support, e.g., a high surface area alumina coating. The support is carried on a suitable carrier such as a monolithic carrier comprising a refractory ceramic or metal honeycomb structure, or refractory particles such as spheres or short, extruded segments of a suitable refractory material.
The high surface area alumina materials, loosely referred to in the art as "gamma alumina" or "activated alumina", typically exhibit a BET surface area in excess of 60 square meters per gram ("m.sup.2 /g"), often up to about 200 m.sup.2 /g or more. Such activated alumina is usually a mixture of the gamma and delta phases of alumina, but may also contain substantial amounts of eta, kappa and theta alumina phases.
In a moving vehicle, exhaust gas temperatures can reach 1000.degree. C., and such elevated temperatures cause the activated alumina (or other) support material to undergo thermal degradation caused by a phase transition with accompanying volume shrinkage, especially in the presence of steam, whereby the catalytic metal becomes occluded in the shrunken support medium with a loss of exposed catalyst surface area and a corresponding decrease in catalytic activity. It is a known expedient in the art to stabilize alumina supports against such thermal degradation by the use of materials such as zirconia, titania, alkaline earth metal oxides such as baria, calcia or strontia or, most usually, rare earth metal oxides, for example, ceria, lanthana and mixtures of two or more rare earth metal oxides For example, see C. D. Keith et al U.S. Pat. No. 4,171,288.
One of the problems faced by present-day catalysts is the high operating temperatures engendered by smaller automotive engines and high speed highway driving. Not only alumina support materials, but other support materials and certain promoter components are especially susceptible to thermal degradation at such high temperatures. Thermal degradation adversely affects the stability of the catalyst and effectiveness of the precious metals used therein. In addition, attempts to improve fuel economy by using A/F ratios higher than stoichiometric, and/or fuel shut-off features, generate a lean (oxygen-rich) exhaust. High exhaust gas temperatures and lean gas conditions accelerate the deterioration of platinum and rhodium catalysts, inasmuch as platinum is more readily sintered, and rhodium more strongly interacts with support materials such as alumina, at such conditions.
For these and other reasons, it is known to utilize refractory metal oxides other than activated alumina as a support for at least some of the catalytic components in a given catalyst. For example, bulk ceria, zirconia, alpha alumina and other materials are known for such use. Although many of these materials suffer from the disadvantage of having a considerably lower BET surface area than activated alumina, that advantage tends to be offset by a greater durability of the resulting catalyst.
It is also conventional wisdom in the art not to disperse rhodium on a rare earth oxide support such as ceria because of the tendency of rhodium to react with the ceria in a manner which diminishes or reduces the catalytic effectiveness of the expensive rhodium. This is especially true for a catalyst intended for use in lean, high-temperature applications, such as those encountered with the relatively high A/F ratios often employed to increase fuel mileage and reduce HC and CO pollution because such conditions promote the undesired rhodium-ceria reaction. In this regard, see U.S. Pat. No. 4,678,770 of C. Z. Wan et al.
It is known that bulk cerium oxide (ceria) provides an excellent refractory oxide support for platinum group metals other than rhodium, and enables the attainment of highly dispersed, small crystallites of platinum on the ceria particles, and that the bulk ceria may be stabilized by impregnation with a solution of an aluminum compound, followed by calcination. For example, see U.S. Pat. No. 4,714,694 of C. Z. Wan et al, which discloses aluminum-stabilized bulk ceria, optionally combined with an activated alumina, to serve as a refractory oxide support for platinum group metal components impregnated thereon. The use of bulk ceria as a catalyst support for platinum group metal catalysts other than rhodium, is also disclosed in U.S. Pat. No. 4,727,052 of C. Z. Wan et al and in U.S. Pat. No. 4,708,946 of Ohata et al.
Japanese Patent J6 3205-141-A discloses a layered automotive catalyst in which the bottom layer comprises platinum or platinum and rhodium dispersed on an alumina support containing rare earth oxides, and a top coat which comprises Pd and Rh dispersed on a support comprising alumina, zirconia and rare earth oxides.
Japanese Patent J6 3077-544-A discloses a layered automotive catalyst having a first layer comprising Pd dispersed on a support comprising alumina, lanthana and other rare earth oxides and a second coat comprising rhodium dispersed on a support comprising alumina, zirconia, lanthana and rare earth oxides.
Japanese Patent J6 3007-845-A discloses an exhaust gas catalyst comprising two catalytic components, one comprising platinum dispersed on a refractory inorganic oxide support and a second comprising Pd and Rh dispersed on a refractory inorganic oxide support.
Japanese Patent J6 1157-346-A discloses an automotive catalyst comprising YtO and platinum group metals dispersed on an alumina-zirconia washcoat.
European Patent Application Number 89303729.1, published Oct. 18, 1989 discloses an exhaust purifying catalyst comprising rhodium and palladium dispersed on a support comprising an alumina and iron oxide, each impregnated with ceria (see Example 13, page 9, lines 60-65).